model pulling apart when skinning
I'm not sure though why that happened. But  you can join the sleeve and stitches with ctrl+J in object mode if they are sperate object. Or select every thing,  go to edit mode and search for (merge by distace) option (F3). And apply it.  It will join the vertices which almost share the same place.
Remember  after doing this you have re assign the weight.
Hope that helps.